1. Field of the Invention
The invention relates generally to the field of analysis of well logs. More specifically, the invention relates to method for quantifying hydrocarbon volumes in laminated subsurface formations that have relatively small lamination thickness using well log data having relatively coarse resolution in the thickness direction.
2. Background Art
Well logs are used to make quantitative estimates of volume of hydrocarbon present in subsurface Earth formations. Well logs are obtained by moving various types of instruments having sensors therein along a wellbore drilled through the subsurface formations. Sensors in typical well log instruments make measurements of particular petrophysical properties of the subsurface formations, including, for example, electrical resistivity, acoustic velocity, density, natural gamma radiation, neutron porosity and dielectric constant, among others.
The sensors in the various types of well logging instruments have different axial resolution (resolution along the direction of the wellbore) and lateral depth of investigation (investigation transverse to the direction of the wellbore) into the formations surrounding the wellbore. As a general principle, greater lateral depth of investigation corresponds to coarser axial resolution. For certain types of measurements, having larger lateral depth of investigation is important because drilling a wellbore may result in displacement of fluids naturally present in the subsurface formations by the liquid phase of fluid used to drill the wellbore. Thus, for certain analyses, including determining fractional rock volume occupied by hydrocarbons, it is necessary to obtain measurements of electrical resistivity that are closely related to the resistivity at such lateral depth from the wellbore as the formation is essentially undisturbed by movement of wellbore fluid into the formation. Obtaining such laterally deep measurements unavoidably results in measurements having relatively coarse axial resolution.
Fine axial resolution is important in particular because some formations consist of a plurality of relatively thin (the thickness of a formation layer generally being defined with respect to a line transverse to the plane of the formation layer) laminations of hydrocarbon bearing (and thus typically hydrocarbon productive) rock formation interleaved with laminations of clay bearing (and consequently substantially impermeable and non-productive) rock formation.
Resistivity measuring devices known in the art may have lateral depth of investigation on the order of 1 to 3, meters from the axis of the instrument. Correspondingly, the axial resolution of such instruments is about 1 to 3 meters. Other devices, such as density and neutron porosity devices, may have axial resolution and lateral investigation depth of about 1/10 to ½ meter. Dielectric constant measuring devices, for example, may have axial resolution and lateral depth of investigation on the order of 1/10 meter or less. Acoustic velocity measuring devices may have axial resolution on the order of 1/10 to 1 meter.
Another resistivity measuring device, for which services are sold by the assignee of the present invention under the trademark RTSCANNER, provides estimates of resistivity of the formation in a direction along the direction of the layers of the formations and in a direction transverse to the layer direction. These resistivity values are referred to as horizontal and vertical resistivities respectively in the description of the present invention. Such device has proven useful in identifying subsurface formations consisting of the alternating productive and non-productive layers as described above. However, the relatively coarse axial resolution of the measurements provided by the foregoing instrument has limited the accuracy of quantitative evaluation of such laminated formations. There continues to be a need for methods of interpreting well log data to obtain quantitative estimates of hydrocarbon volume in place where the subsurface formations include thin laminations.